Localise simultaneously in previous and current map

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How can we localise in a real time map (similar to SLAM) as well as in a previously created saved map? It is something like combining amcl and gmapping to work together(although they don't work together). The previously created map would be of the same environment but might be of only half of the actual run which the robot makes in the second run.

If that is not possible is it possible to combine the previously created map (assuming I dont have any other information about the map except for a map.pgm file itself) with the real time map and then create a single occupancy grid in which amcl can easily localise.